In recent years, along with reductions in sizes of portable electronic devices and with development of hybrid automobiles, electric automobiles, etc. giving consideration to environmental problems such as atmospheric pollution and carbon dioxide increase, there is an increasing demand for secondary batteries applicable to such electronic devices and electric automobiles having excellent characteristics such as high efficiency, high output, high energy density, and light weight. Various studies have been conducted for providing a secondary battery having such required characteristics.
For example, a lithium-ion secondary battery usually has a structure in which a space between a positive electrode (cathode) and a negative electrode (anode) is filled with an electrolytic solution composed of a lithium salt, such as LiPF6, dissolved in a non-aqueous organic solvent. Lithium transition metal oxide is used as the positive electrode, and lithium or carbon (graphite) is mainly used as the negative electrode. The electrolytic solution has superior ionic conductivity and negligible electrical conductivity. During charging, lithium ions move from the positive electrode to the negative electrode, and during discharging, lithium ions move in the reverse direction.
The positive electrode and the negative electrode of the lithium-ion secondary battery are separated from each other with a separator formed of a porous polymer film and are in a structure preventing their direct electric contact. Accordingly, the separator for a secondary battery is required to have various characteristics, such as film thickness (thinness), mechanical strength, ionic conductance (during containing of an electrolytic solution), electric insulation, electrolytic solution resistance, electrolytic solution-retaining property, and wettability. As the separator for a secondary battery having these properties, a porous film made of polyolefins, such as polyethylene and polypropylene, is generally used. Porous films have random pores at a porosity of about 35% to 40% are widely used as separators for lithium-ion secondary batteries having negative electrodes of carbon.
It is known that in the case of using these conventionally known porous separators, lithium metal is deposited on negative electrodes after repeated charge and discharge cycles. Furthermore, the repetition of charge and discharge of a battery is known to cause the growth of dendrite lithium, resulting in a short circuit of the battery and this problem needs to be solved (Patent Document 1).
In addition, the separator itself is also known to deposit ions of the metal constituting the positive electrode due to the pore diameter distribution, inhibiting ion transfer in the electrolytic solution and causing the growth of dendrite, resulting in a short circuit of the battery (Patent Document 2). On the other hand, a separator having few large diameter pores and having a uniform pore diameter has a problem of increase in manufacturing cost, and a separator having large diameter pores suffers from a lack of mechanical strength leading to a problem of deterioration in handling properties.
Patent Document 1: Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2010-537387
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2008-166212